Ethnic violence has again erupted in South Sudan and up to 50,000 people have fled from the town of Pibor in the Jonglei state.

As feared, a legion of 6,000 Lou Nuer fighters attacked Pibor on Monday, sending locals running from their homes.

We are worried about their conditions. They are without water, shelter and food. They are hiding in the bush. I think it is between 20,000 and 50,000. This is an estimate only, Lise Grande, U.N. humanitarian coordinator for South Sudan, told Reuters.

The Lou Nuer tribesmen attacked their Murle rivals again on Saturday, and have continued to burn down homes and steal cattle, both group's traditional source of income. The Nuer warriors are reportedly following displaced Murle and are attacking women and children.

Several flanks of the attackers have moved in a south-easterly direction [from Pibor], almost certainly looking for cattle, Grande told the BBC.

Last week, the United Nations sent a battalion of combat-ready peacekeepers to Pibor to try to prevent further violence, but the troops, as well as South Sudanese police and soldiers, are outnumbered and have been unable to stop the warring clans.

The U.N. troops are there but they are not fighting the fighters who entered the town, Rev. Orozu Lokine Daky of Pibor's Presbyterian Church told the BBC.

They are just trying to protect the city center only, the rest of the town is now under [the control of] the fighters.

The situation is deteriorating. My own mother and my own sister are now missing. I don't know where they are. I assume they are dead, he added.

The government of South Sudan said it is sending more troops and police officers to Pibor. Youth Nuer fighters have already vowed to fight the United Nations and Sudan People's Liberation Army soldiers if necessary.

The recent conflict between the Nuer and the Murle began after a number of cattle raids in June. In the past few months, an estimated 1,000 people have been killed.